About inclined underwater hull form

Discussion in 'Hydrodynamics and Aerodynamics' started by fredschmidt, Feb 18, 2012.

  1. yipster
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    yipster designer

    leo, mikko, all: as you probably know but no harm to mention here again
    is a delta gives a different type of lift: http://en.wikipedia.org/wiki/Vortex_lift
    and probably requires another approach in cfd i asume
     
  2. Leo Lazauskas
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    Leo Lazauskas Senior Member

    Thanks, Yipster. My report includes predictions with "vortex lift" and without.
    Whether the method is valid is debatable, but it certainly improves agreement
    with experimental values for many of the wings.
     
  3. daiquiri
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    daiquiri Engineering and Design

    Let me put another dish on the table, for the sake of discussion... ;)

    There are some traditional boats which intensively use the chine-produced vortex lift. Take a look at this "batana" (or "batela") fishing boat from northern Adriatic sea:

    [​IMG]

    Another photo of batana under sails here: http://2.bp.blogspot.com/-rKQ3zgcZ09A/Tt3TJKK6UkI/AAAAAAAAAZk/dph9RvyiaCk/s1600/batana.jpg
    Notice how far aft is the sail. The mast is approximately at 35-40% of the hull length from the transom, and the boom extends well beyond the transom. In this way, the large central area of the boat is left free of rigging and serves as a workspace. It has a huge rudder blade in the water, which alone cannot efficiently balance the yawing moment from the sails. It needs a help from the leeward chine, as shown in the drawing attached below. A chine doesn't need to produce a big force. A small lift contribution is enough to make the system of lateral forces balanced, because it has a good lever arm. These old boats don't use a keel because they were built for simplicity and economy, and occasionally they had to be hauled on the beach. This is by far the simplest way of making them work their way upwind.

    The "system" is also dynamically course-stable (in the engineering, not NA's, sense ;) ) because any perturbance which would momentarily increase the leeway angle (AoA of the chine) or wind apparent angle, would also shift the center of pressure of the chine towards the transom, thus creating a yawing moment which would tend to bring the boat back into the initial configuration. So simple. :)

    So, with this example in mind, one can easily imagine how the chine of Star-class sailboats give a contribution to the overall system of forces. However, a Star has a keel too (besides the rudder), so the role played by chines in the lift generation is imho much less important here than it is for the old keel-less dinghies like batana and alikes.

    Cheers
     

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    Last edited: Mar 25, 2012
  4. Remmlinger
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    Remmlinger engineer

    One can estimate the lift generated by the chine (see post #75) and compare it with the lift of the foil. That is better than guessing. The force calculated according to slender wing theory agrees reasonably well with tank test data.
    Uli
     
  5. DCockey
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    DCockey Senior Member

    A RANS code along with a suitable grid can calculate lift of a surface with vortices from the edges. Emphasis on suitable grid which means sufficient refinement. My guess is for "vortex lift" the grid will be considerably more important then the details of the turbulence modeling, etc. In fact an inviscid Euler code sufficient "damping" would probably give reasonable results with a suitable grid.

    I've also seen "vortex lift" modeled using an invisicid panel code with the shear layers modeled with panels and the shape of the shear layers determined during the solution using appropriate boundary conditions. It's difficult to do and the amount of computations needed probably equal or exceed those needed by a field solver.
     
  6. daiquiri
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    daiquiri Engineering and Design

    I have done that homework and have found out that, according to the slender wing theory, letting the hull produce any lift is disadvantageous in terms of induced drag.
    Considering two cases:
    1) all the lift is produced by a fin keel having dimensions C (chord) and T (draft)
    2) part of the lift is due to a full-length keel (or a very sharp chine) having a draft t, and the rest of the lift is produced by the fin keel;
    the angle of attack in the second case is lower than the AoA from the first case, because the chine-keel has relieved some load from the fin keel. The relationship is:
    AoA,2 = AoA,1 / {1 + [t / (C*T)^2]}

    But... the induced drag in the second case is 1+(t/C)^2 times higher than the iduced drag in the first case:
    Di,2 = Di,1 * [1 + (t/C)^2]

    The total area in both cases has been kept constant, as were the total draft T and lift.

    That's a theoretical result, valid for low speeds and low heel angles only. As it is 2 a.m. here, the results need a confirmation from an independent re-calculation done by another volunteer, before being taken as valid and verified. :)
    Good night folks.
     
  7. Remmlinger
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    Remmlinger engineer

    slender wing theory

    The method of R. T. Jones applies to Delta wings with low aspect ratio. The induces drag factor CDi is:
    CDi = CL * alpha / 2 - if leading-edge suction occurs and
    CDi = CL * alpha - for a sharp leading edge without suction force

    Uli
     
  8. daiquiri
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    daiquiri Engineering and Design

    Thanks for the general theory. How about showing some practical results for this particular case? For example, cases 1) and 2) in my previous post.
     
  9. DCockey
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    DCockey Senior Member

    What theory did you use for this analysis?
     
  10. daiquiri
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    daiquiri Engineering and Design

    Jones' theory for the low-AR full-length keel and Prandtl lifting-line theory with elliptical loading case for the high-AR fin keel. But It was 2 a.m., had just finished a ton of a paying jobs and the errors were inevitable. There is actually at least one error in the calculation, since I have used a rectangular plate model for the cine-keel. It should have been a triangle, as Remmlinger has rightly pointed out. Will redo the calcs when I find some time to do it. Or you could do it as well, if you wish. Just remember, we need indications here, not exact values. The exact values are beyond these simple theoretical models.
    Cheers
     
  11. Remmlinger
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    Remmlinger engineer

    For comparison one can use a foil with an elliptical load distribution. In this case Helmbolds equation applies and the induced drag coefficient is:

    CDi = CL * alpha / (SQRT(.25*AR^2+1)+1)
    AR = Aspect ratio of foil, including mirror image = 2*span/chord
    compare this with the sharp edge chine:
    CDi = CL * alpha

    even for moderate aspect rations the foil produces the lift with a much smaller induced drag penalty.

    Uli
     
  12. DCockey
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    DCockey Senior Member

    And if lift and the lift distribution are kept constant for "non-slender" wing the induced drag depends on draft/span only.

    For an elliptical lift distribution: 1 / [Pi * (0.5 * Density of Water * Speed ^ 2)] *[ (Lift / Span) ^ 2] Myth of Aspect Ratio
     
  13. philSweet
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    philSweet Senior Member

    Ok, I've just read thread start to finish. I'm unabashedly fond of chines in small boats-Stars, Lightnings, Mac Dinghies, Y fliers, Windmills, etc.

    So some arguments why chines might be more favored in small boats compared to their larger cousins.

    Smaller boats are dimensionally challenged with respect to righting moments and must make up ground relative to their larger cousins with increased D/L, B/L, and draft, if externally ballasted. Lesser aspect ratios produce smaller heeling moments, so the draft thing gets weird if externally ballasted.

    Hull lift can be very important. I built a 16' Mac Dingy-like boat that could sail to windward with boards up in 9" of water. I was running most of a Hobie 16 main and a Hobie 18 jib, so not lightly powered for a skiff. The chines were just at the waterline lightship.

    A few observations-

    Daiquiri touched on this but I think it needs a foot stomper- THE L/D OF THE HULL DOESN'T MATTER. What matters is the amount of lift you can get from a given change in drag. It's a trade off, and the relevant comparison is the rate of change in drag between the hull and the keel.

    The second matter is the moment arm as far as heeling is concerned. Hull lift is a better sort of lift than keel lift because it contributes less to the heeling moment. There isn't a simple way to characterize this advantage overall, but maybe for good hulls with a L/B of 3 you could make some generalities. (basically, you want to compare hulls which have a similar percentage of wave drag to friction drag, so similar in power) I think small boats will better exploit hull lift because they will typically run lower aspect ratio foils (but perhaps relatively deeper ones, ie a lot more foil area than their larger cousins if the crew makes for a higher percentage of movable ballast. Although this is a non issue for IOM.)

    Like size, boat speed cannot be factored out. Chines should be somewhat better as speed decreases. Perturbations become more pronounced as speed drops and low aspect keels (chines) have a different stall characteristic than foils.

    Another consequence of mechanical similitude is that a small boat will probably sail at a higher leeway and tack through a larger angle than a larger cousin. This also gives a chine a bit better chance on the smaller boat.

    So why a chine on a smaller boat? Compared to a larger boat, you reduce aspect ratio and increase B/L

    1. A chine can reduce waterline beam for a given RM. Wave drag is roughly proportional to B^2. Smaller boats need a proportionately bigger beam, so a chine has a better chance on a smaller boat.

    2. Smaller boats tend to run lower aspect sail plans to reduce heeling moment-or at least to not increase heeling moment and still provide head clearance under the boom. Smaller aspect ratio foils also for same reason, however, foils may need to be a higher percentage of area if crew hiking is more important. Smaller boats sail slower. All of the above lead to optimizing windward performance at a higher leeway angle and a larger tack angle, and this favors a chine compared to a boat with smaller leeway angles.

    The interaction between the fin and the surface is an interesting problem. With a lower aspect foil and a higher B/L, should be a bigger factor in a small boat. A higher D/L could offset the problem. Should smaller keel boats be set up for less heel than their larger cousins? (I like twin asymmetric bilge boards, And I think they improve the chine effect, but it's just a feeling)
     
    Last edited: Mar 24, 2012
  14. Earl Boebert
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    Earl Boebert Senior Member

    Correction to Rip Tide Lines

    Roger Adams, the USVMYG member who captured the lines of Rip Tide, informs me that there was small error in the lines I posted earlier in this thread. The table of offsets is correct; only the other formats were affected. The corrected lines are attached. I don't think the nature of the error will affect any of the previous discussions.

    Cheers,

    Earl
     

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  15. Mikko Brummer
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    Mikko Brummer Senior Member

    RANS-code can handle vortex lift easily, as seen in these Dhow simulations.
     

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